At the present time, the construction of motor vehicles has become remarkably electronic. Instruments, including, among other things, the engine, installed in each section of a motor vehicle are interconnected to each other via a control computer so that complex operations can be performed.
In such case, even if a malfunction of a certain single installed instrument is detected, often the true cause for the malfunction, including the interrelation with other installed instruments, cannot be determined unless a wide range of data (diagnostic data) indicating the state of the motor vehicle at the time the malfunction is detected is collected. Also, after a temporary malfunction, there is a possibility that the malfunction will be corrected naturally. Further, often this is a sign that a complete failure will occur; however, it is quite difficult to find the cause thereof by performing an inspection after getting out of the motor vehicle.
Accordingly, a self-diagnosing apparatus is disclosed in Japanese Patent Laid-Open Nos. 62-142849 and 63-90738, in which diagnostic data from each section of a motor vehicle is updated and stored in a memory where the contents are stored at specified intervals even when the power supply is shut down; updating of the contents of the memory being inhibited (frozen) after a malfunction of an installed instrument is detected, so that diagnostic data at the time the malfunction is detected will be stored and the cause of the malfunction can be determined accurately after getting out of the motor vehicle.
Not all malfunctions of the installed instruments in a motor vehicle have the same priority. For example, malfunctions such as a misfire in the engine, a malfunction of the fuel system and the like are serious malfunctions, as such have a higher priority than malfunctions. Therefore, even if other malfunctions having respectively lower priorities have occured earlier, when a malfunction having a higher priority occurs, thereafter, diagnostic data obtained at the time of the higher priority malfunction must be held in precedence over other data related to the lower priority malfunction.
However, in the above-described conventional apparatus, when diagnostic data is frozen once when a malfunction of the apparatus occurs, such diagnostic data is not updated even when a malfunction having a priority higher than that of the above one occurs thereafter.
Therefore, when a malfunction having a high priority is detected, it is conceivable that the diagnostic data is replaced forcibly. However, if an ignition switch is turned off while this data is being replaced, a portion of the data which has not yet been replaced remains in a part of the memory and, on the contrary, erroneous diagnostic data is given, which is problematical. It is conceivable provide a main relay for supplying power for some time after the ignition switch has been turned off. However, such construction results in the hardware becoming overly complex.
The present invention solves the above-described problems of the prior art. It is an object of the present invention to provide a self-diagnosing apparatus for motor vehicles which is capable of reliably storing diagnostic data on high priority malfunctions.